Method for processing a packet data convergence protocol service data unit at a user equipment in a dual connectivity system and device therefor

ABSTRACT

The present invention relates to a wireless communication system. More specifically, the present invention relates to a method and a device for processing a PDCP SDU in a dual connectivity system, the method comprising: receiving a PDCP SDU (Packet Data Convergence Protocol Service Data Unit) from a upper layer; starting a timer associated with the PDCP SDU; checking whether a certain condition related to the PDCP SDU process is satisfied or not if the timer expires; and discarding the PDCP SDU if the certain condition related to the PDCP SDU process is satisfied.

TECHNICAL FIELD

The present invention relates to a wireless communication system and,more particularly, to a method for processing a PDCP SDU at a UE in adual connectivity system and a device therefor.

BACKGROUND ART

As an example of a mobile communication system to which the presentinvention is applicable, a 3rd Generation Partnership Project Long TermEvolution (hereinafter, referred to as LTE) communication system isdescribed in brief.

FIG. 1 is a view schematically illustrating a network structure of anE-UMTS as an exemplary radio communication system. An Evolved UniversalMobile Telecommunications System (E-UMTS) is an advanced version of aconventional Universal Mobile Telecommunications System (UMTS) and basicstandardization thereof is currently underway in the 3GPP. E-UMTS may begenerally referred to as a Long Term Evolution (LTE) system. For detailsof the technical specifications of the UMTS and E-UMTS, reference can bemade to Release 7 and Release 8 of “3rd Generation Partnership Project;Technical Specification Group Radio Access Network”.

Referring to FIG. 1, the E-UMTS includes a User Equipment (UE), eNode Bs(eNBs), and an Access Gateway (AG) which is located at an end of thenetwork (E-UTRAN) and connected to an external network. The eNBs maysimultaneously transmit multiple data streams for a broadcast service, amulticast service, and/or a unicast service.

One or more cells may exist per eNB. The cell is set to operate in oneof bandwidths such as 1.25, 2.5, 5, 10, 15, and 20 MHz and provides adownlink (DL) or uplink (UL) transmission service to a plurality of UEsin the bandwidth. Different cells may be set to provide differentbandwidths. The eNB controls data transmission or reception to and froma plurality of UEs. The eNB transmits DL scheduling information of DLdata to a corresponding UE so as to inform the UE of a time/frequencydomain in which the DL data is supposed to be transmitted, coding, adata size, and hybrid automatic repeat and request (HARQ)-relatedinformation. In addition, the eNB transmits UL scheduling information ofUL data to a corresponding UE so as to inform the UE of a time/frequencydomain which may be used by the UE, coding, a data size, andHARQ-related information. An interface for transmitting user traffic orcontrol traffic may be used between eNBs. A core network (CN) mayinclude the AG and a network node or the like for user registration ofUEs. The AG manages the mobility of a UE on a tracking area (TA) basis.One TA includes a plurality of cells.

Although wireless communication technology has been developed to LTEbased on wideband code division multiple access (WCDMA), the demands andexpectations of users and service providers are on the rise. Inaddition, considering other radio access technologies under development,new technological evolution is required to secure high competitivenessin the future. Decrease in cost per bit, increase in serviceavailability, flexible use of frequency bands, a simplified structure,an open interface, appropriate power consumption of UEs, and the likeare required.

DISCLOSURE Technical Problem

An object of the present invention devised to solve the problem lies ina method and device for processing a PDCP SDU at a UE in a dualconnectivity system. The technical problems solved by the presentinvention are not limited to the above technical problems and thoseskilled in the art may understand other technical problems from thefollowing description.

Technical Solution

The object of the present invention can be achieved by providing amethod for a User Equipment (UE) operating in a wireless communicationsystem, the method comprising: receiving a PDCP SDU (Packet DataConvergence Protocol Service Data Unit) from a upper layer; starting atimer associated with the PDCP SDU; checking whether a certain conditionrelated to the PDCP SDU process is satisfied or not if the timerexpires; and discarding the PDCP SDU if the certain condition related tothe PDCP SDU process is satisfied.

In another aspect of the present invention, provided herein is a UE(User Equipment) for performing random access procedure in a wirelesscommunication system, the UE comprising: an RF (Radio Frequency) module;and a processor configured to control the RF module, wherein theprocessor is configured to receive a PDCP SDU (Packet Data ConvergenceProtocol Service Data Unit) from a upper layer, to start a timerassociated with the PDCP SDU, to check whether a certain conditionrelated to the PDCP SDU process is satisfied or not when the timerexpires, and to discard the PDCP SDU if the certain condition related tothe PDCP SDU process is satisfied.

Preferably, wherein the certain condition related to the PDCP SDUprocess is satisfied if the PDCP sequence number is not associated withthe PDCP SDU when the timer expire.

Preferably, the method further comprise: generating a PDCP PDU (ProtocolData Unit) using the PDCP SDU and the PDCP Sequence Number associatedwith the PDCP SDU, if the PDCP Sequence Number is associated with thePDCP SDU when the timer expires; and submitting the generated PDCP PDUto a lower layer.

Preferably, the certain condition related to the PDCP SDU process issatisfied if a PDCP PDU (Protocol Data Unit) including the PDCP SDU andPDCP SN associated with the PDCP SDU is not generated when the timerexpires.

Preferably, the method further comprise: performing a re-numbering ofconsecutive PDCP SDUs that follow the discarded PDCP SDU so that thereis no PDCP Sequence Number gap between the consecutive PDCP SDUs

Preferably, the certain condition related to the PDCP SDU process issatisfied if the PDCP PDU (Protocol Data Unit) including the PDCP SDUand PDCP SN associated with the PDCP SDU is not submitted to the lowerlayer when the timer expires.

Preferably, the method further comprise: performing a re-numbering ofconsecutive PDCP SDUs that follow the discarded PDCP SDU so that thereis no PDCP Sequence Number gap between the consecutive PDCP SDUs.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

Advantageous Effects

According to the present invention, processing PDCP SDU can beefficiently performed in a dual connectivity system. It will beappreciated by persons skilled in the art that that the effects achievedby the present invention are not limited to what has been particularlydescribed hereinabove and other advantages of the present invention willbe more clearly understood from the following detailed description takenin conjunction with the accompanying drawings.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the invention andtogether with the description serve to explain the principle of theinvention.

FIG. 1 is a diagram showing a network structure of an Evolved UniversalMobile Telecommunications System (E-UMTS) as an example of a wirelesscommunication system;

FIG. 2A is a block diagram illustrating network structure of an evolveduniversal mobile telecommunication system (E-UMTS), and FIG. 2B is ablock diagram depicting architecture of a typical E-UTRAN and a typicalEPC;

FIG. 3 is a diagram showing a control plane and a user plane of a radiointerface protocol between a UE and an E-UTRAN based on a 3rd generationpartnership project (3GPP) radio access network standard;

FIG. 4 is a diagram of an example physical channel structure used in anE-UMTS system;

FIG. 5 is a block diagram of a communication apparatus according to anembodiment of the present invention;

FIG. 6 is a diagram for carrier aggregation;

FIG. 7 is a conceptual diagram for dual connectivity between a MasterCell Group (MCS) and a Secondary Cell Group (SCG);

FIG. 8a is a conceptual diagram for C-Plane connectivity of basestations involved in dual connectivity, and FIG. 8b is a conceptualdiagram for U-Plane connectivity of base stations involved in dualconnectivity;

FIG. 9 is a conceptual diagram for radio protocol architecture for dualconnectivity;

FIG. 10 is a diagram for a general overview of the LTE protocolarchitecture for the downlink;

FIG. 11 is a conceptual diagram for a PDCP entity architecture;

FIG. 12 is a conceptual diagram for functional view of a PDCP entity;and

FIGS. 13A to 13D are conceptual diagrams for processing a PDCP SDU in adual connectivity system according to embodiments of the presentinvention.

BEST MODE

Universal mobile telecommunications system (UMTS) is a 3rd Generation(3G) asynchronous mobile communication system operating in wideband codedivision multiple access (WCDMA) based on European systems, globalsystem for mobile communications (GSM) and general packet radio services(GPRS). The long-term evolution (LTE) of UMTS is under discussion by the3rd generation partnership project (3GPP) that standardized UMTS.

The 3GPP LTE is a technology for enabling high-speed packetcommunications. Many schemes have been proposed for the LTE objectiveincluding those that aim to reduce user and provider costs, improveservice quality, and expand and improve coverage and system capacity.The 3G LTE requires reduced cost per bit, increased serviceavailability, flexible use of a frequency band, a simple structure, anopen interface, and adequate power consumption of a terminal as anupper-level requirement.

Hereinafter, structures, operations, and other features of the presentinvention will be readily understood from the embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings. Embodiments described later are examples in which technicalfeatures of the present invention are applied to a 3GPP system.

Although the embodiments of the present invention are described using along term evolution (LTE) system and a LTE-advanced (LTE-A) system inthe present specification, they are purely exemplary. Therefore, theembodiments of the present invention are applicable to any othercommunication system corresponding to the above definition. In addition,although the embodiments of the present invention are described based ona frequency division duplex (FDD) scheme in the present specification,the embodiments of the present invention may be easily modified andapplied to a half-duplex FDD (H-FDD) scheme or a time division duplex(TDD) scheme.

FIG. 2A is a block diagram illustrating network structure of an evolveduniversal mobile telecommunication system (E-UMTS). The E-UMTS may bealso referred to as an LTE system. The communication network is widelydeployed to provide a variety of communication services such as voice(VoIP) through IMS and packet data.

As illustrated in FIG. 2A, the E-UMTS network includes an evolved UMTSterrestrial radio access network (E-UTRAN), an Evolved Packet Core (EPC)and one or more user equipment. The E-UTRAN may include one or moreevolved NodeB (eNodeB) 20, and a plurality of user equipment (UE) 10 maybe located in one cell. One or more E-UTRAN mobility management entity(MME)/system architecture evolution (SAE) gateways 30 may be positionedat the end of the network and connected to an external network.

As used herein, “downlink” refers to communication from eNodeB 20 to UE10, and “uplink” refers to communication from the UE to an eNodeB. UE 10refers to communication equipment carried by a user and may be alsoreferred to as a mobile station (MS), a user terminal (UT), a subscriberstation (SS) or a wireless device.

FIG. 2B is a block diagram depicting architecture of a typical E-UTRANand a typical EPC.

As illustrated in FIG. 2B, an eNodeB 20 provides end points of a userplane and a control plane to the UE 10. MME/SAE gateway 30 provides anend point of a session and mobility management function for UE 10. TheeNodeB and MME/SAE gateway may be connected via an S1 interface.

The eNodeB 20 is generally a fixed station that communicates with a UE10, and may also be referred to as a base station (BS) or an accesspoint. One eNodeB 20 may be deployed per cell. An interface fortransmitting user traffic or control traffic may be used between eNodeBs20.

The MME provides various functions including NAS signaling to eNodeBs20, NAS signaling security, AS Security control, Inter CN node signalingfor mobility between 3GPP access networks, Idle mode UE Reachability(including control and execution of paging retransmission), TrackingArea list management (for UE in idle and active mode), PDN GW andServing GW selection, MME selection for handovers with MME change, SGSNselection for handovers to 2G or 3G 3GPP access networks, Roaming,Authentication, Bearer management functions including dedicated bearerestablishment, Support for PWS (which includes ETWS and CMAS) messagetransmission. The SAE gateway host provides assorted functions includingPer-user based packet filtering (by e.g. deep packet inspection), LawfulInterception, UE IP address allocation, Transport level packet markingin the downlink, UL and DL service level charging, gating and rateenforcement, DL rate enforcement based on APN-AMBR. For clarity MME/SAEgateway 30 will be referred to herein simply as a “gateway,” but it isunderstood that this entity includes both an MME and an SAE gateway.

A plurality of nodes may be connected between eNodeB 20 and gateway 30via the S1 interface. The eNodeBs 20 may be connected to each other viaan X2 interface and neighboring eNodeBs may have a meshed networkstructure that has the X2 interface.

As illustrated, eNodeB 20 may perform functions of selection for gateway30, routing toward the gateway during a Radio Resource Control (RRC)activation, scheduling and transmitting of paging messages, schedulingand transmitting of Broadcast Channel (BCCH) information, dynamicallocation of resources to UEs 10 in both uplink and downlink,configuration and provisioning of eNodeB measurements, radio bearercontrol, radio admission control (RAC), and connection mobility controlin LTE_ACTIVE state. In the EPC, and as noted above, gateway 30 mayperform functions of paging origination, LTE-IDLE state management,ciphering of the user plane, System Architecture Evolution (SAE) bearercontrol, and ciphering and integrity protection of Non-Access Stratum(NAS) signaling.

The EPC includes a mobility management entity (MME), a serving-gateway(S-GW), and a packet data network-gateway (PDN-GW). The MME hasinformation about connections and capabilities of UEs, mainly for use inmanaging the mobility of the UEs. The S-GW is a gateway having theE-UTRAN as an end point, and the PDN-GW is a gateway having a packetdata network (PDN) as an end point.

FIG. 3 is a diagram showing a control plane and a user plane of a radiointerface protocol between a UE and an E-UTRAN based on a 3GPP radioaccess network standard. The control plane refers to a path used fortransmitting control messages used for managing a call between the UEand the E-UTRAN. The user plane refers to a path used for transmittingdata generated in an application layer, e.g., voice data or Internetpacket data.

A physical (PHY) layer of a first layer provides an information transferservice to a higher layer using a physical channel. The PHY layer isconnected to a medium access control (MAC) layer located on the higherlayer via a transport channel. Data is transported between the MAC layerand the PHY layer via the transport channel. Data is transported betweena physical layer of a transmitting side and a physical layer of areceiving side via physical channels. The physical channels use time andfrequency as radio resources. In detail, the physical channel ismodulated using an orthogonal frequency division multiple access (OFDMA)scheme in downlink and is modulated using a single carrier frequencydivision multiple access (SC-FDMA) scheme in uplink.

The MAC layer of a second layer provides a service to a radio linkcontrol (RLC) layer of a higher layer via a logical channel. The RLClayer of the second layer supports reliable data transmission. Afunction of the RLC layer may be implemented by a functional block ofthe MAC layer. A packet data convergence protocol (PDCP) layer of thesecond layer performs a header compression function to reduceunnecessary control information for efficient transmission of anInternet protocol (IP) packet such as an IP version 4 (IPv4) packet oran IP version 6 (IPv6) packet in a radio interface having a relativelysmall bandwidth.

A radio resource control (RRC) layer located at the bottom of a thirdlayer is defined only in the control plane. The RRC layer controlslogical channels, transport channels, and physical channels in relationto configuration, re-configuration, and release of radio bearers (RBs).An RB refers to a service that the second layer provides for datatransmission between the UE and the E-UTRAN. To this end, the RRC layerof the UE and the RRC layer of the E-UTRAN exchange RRC messages witheach other.

One cell of the eNB is set to operate in one of bandwidths such as 1.25,2.5, 5, 10, 15, and 20 MHz and provides a downlink or uplinktransmission service to a plurality of UEs in the bandwidth. Differentcells may be set to provide different bandwidths.

Downlink transport channels for transmission of data from the E-UTRAN tothe UE include a broadcast channel (BCH) for transmission of systeminformation, a paging channel (PCH) for transmission of paging messages,and a downlink shared channel (SCH) for transmission of user traffic orcontrol messages. Traffic or control messages of a downlink multicast orbroadcast service may be transmitted through the downlink SCH and mayalso be transmitted through a separate downlink multicast channel (MCH).

Uplink transport channels for transmission of data from the UE to theE-UTRAN include a random access channel (RACH) for transmission ofinitial control messages and an uplink SCH for transmission of usertraffic or control messages. Logical channels that are defined above thetransport channels and mapped to the transport channels include abroadcast control channel (BCCH), a paging control channel (PCCH), acommon control channel (CCCH), a multicast control channel (MCCH), and amulticast traffic channel (MTCH).

FIG. 4 is a view showing an example of a physical channel structure usedin an E-UMTS system. A physical channel includes several subframes on atime axis and several subcarriers on a frequency axis. Here, onesubframe includes a plurality of symbols on the time axis. One subframeincludes a plurality of resource blocks and one resource block includesa plurality of symbols and a plurality of subcarriers. In addition, eachsubframe may use certain subcarriers of certain symbols (e.g., a firstsymbol) of a subframe for a physical downlink control channel (PDCCH),that is, an L1/L2 control channel. In FIG. 4, an L1/L2 controlinformation transmission area (PDCCH) and a data area (PDSCH) are shown.In one embodiment, a radio frame of 10 ms is used and one radio frameincludes 10 subframes. In addition, one subframe includes twoconsecutive slots. The length of one slot may be 0.5 ms. In addition,one subframe includes a plurality of OFDM symbols and a portion (e.g., afirst symbol) of the plurality of OFDM symbols may be used fortransmitting the L1/L2 control information. A transmission time interval(TTI) which is a unit time for transmitting data is 1 ms.

A base station and a UE mostly transmit/receive data via a PDSCH, whichis a physical channel, using a DL-SCH which is a transmission channel,except a certain control signal or certain service data. Informationindicating to which UE (one or a plurality of UEs) PDSCH data istransmitted and how the UE receive and decode PDSCH data is transmittedin a state of being included in the PDCCH.

For example, in one embodiment, a certain PDCCH is CRC-masked with aradio network temporary identity (RNTI) “A” and information about datais transmitted using a radio resource “B” (e.g., a frequency location)and transmission format information “C” (e.g., a transmission blocksize, modulation, coding information or the like) via a certainsubframe. Then, one or more UEs located in a cell monitor the PDCCHusing its RNTI information. And, a specific UE with RNTI “A” reads thePDCCH and then receive the PDSCH indicated by B and C in the PDCCHinformation.

FIG. 5 is a block diagram of a communication apparatus according to anembodiment of the present invention.

The apparatus shown in FIG. 5 can be a user equipment (UE) and/or eNBadapted to perform the above mechanism, but it can be any apparatus forperforming the same operation.

As shown in FIG. 5, the apparatus may comprises a DSP/microprocessor(110) and RF module (transmiceiver; 135). The DSP/microprocessor (110)is electrically connected with the transciver (135) and controls it. Theapparatus may further include power management module (105), battery(155), display (115), keypad (120), SIM card (125), memory device (130),speaker (145) and input device (150), based on its implementation anddesigner's choice.

Specifically, FIG. 5 may represent a UE comprising a receiver (135)configured to receive a request message from a network, and atransmitter (135) configured to transmit the transmission or receptiontiming information to the network. These receiver and the transmittercan constitute the transceiver (135). The UE further comprises aprocessor (110) connected to the transceiver (135: receiver andtransmitter).

Also, FIG. 5 may represent a network apparatus comprising a transmitter(135) configured to transmit a request message to a UE and a receiver(135) configured to receive the transmission or reception timinginformation from the UE. These transmitter and receiver may constitutethe transceiver (135). The network further comprises a processor (110)connected to the transmitter and the receiver. This processor (110) maybe configured to calculate latency based on the transmission orreception timing information.

FIG. 6 is a diagram for carrier aggregation.

Carrier aggregation technology for supporting multiple carriers isdescribed with reference to FIG. 6 as follows. As mentioned in theforegoing description, it may be able to support system bandwidth up tomaximum 100 MHz in a manner of bundling maximum 5 carriers (componentcarriers: CCs) of bandwidth unit (e.g., 20 MHz) defined in a legacywireless communication system (e.g., LTE system) by carrier aggregation.Component carriers used for carrier aggregation may be equal to ordifferent from each other in bandwidth size. And, each of the componentcarriers may have a different frequency band (or center frequency). Thecomponent carriers may exist on contiguous frequency bands. Yet,component carriers existing on non-contiguous frequency bands may beused for carrier aggregation as well. In the carrier aggregationtechnology, bandwidth sizes of uplink and downlink may be allocatedsymmetrically or asymmetrically.

Multiple carriers (component carriers) used for carrier aggregation maybe categorized into primary component carrier (PCC) and secondarycomponent carrier (SCC). The PCC may be called P-cell (primary cell) andthe SCC may be called S-cell (secondary cell). The primary componentcarrier is the carrier used by a base station to exchange traffic andcontrol signaling with a user equipment. In this case, the controlsignaling may include addition of component carrier, setting for primarycomponent carrier, uplink (UL) grant, downlink (DL) assignment and thelike. Although a base station may be able to use a plurality ofcomponent carriers, a user equipment belonging to the corresponding basestation may be set to have one primary component carrier only. If a userequipment operates in a single carrier mode, the primary componentcarrier is used. Hence, in order to be independently used, the primarycomponent carrier should be set to meet all requirements for the dataand control signaling exchange between a base station and a userequipment.

Meanwhile, the secondary component carrier may include an additionalcomponent carrier that can be activated or deactivated in accordancewith a required size of transceived data. The secondary componentcarrier may be set to be used only in accordance with a specific commandand rule received from a base station. In order to support an additionalbandwidth, the secondary component carrier may be set to be usedtogether with the primary component carrier. Through an activatedcomponent carrier, such a control signal as a UL grant, a DL assignmentand the like can be received by a user equipment from a base station.Through an activated component carrier, such a control signal in UL as achannel quality indicator (CQI), a precoding matrix index (PMI), a rankindicator (RI), a sounding reference signal (SRS) and the like can betransmitted to a base station from a user equipment.

Resource allocation to a user equipment can have a range of a primarycomponent carrier and a plurality of secondary component carriers. In amulti-carrier aggregation mode, based on a system load (i.e.,static/dynamic load balancing), a peak data rate or a service qualityrequirement, a system may be able to allocate secondary componentcarriers to DL and/or UL asymmetrically. In using the carrieraggregation technology, the setting of the component carriers may beprovided to a user equipment by a base station after RRC connectionprocedure. In this case, the RRC connection may mean that a radioresource is allocated to a user equipment based on RRC signalingexchanged between an RRC layer of the user equipment and a network viaSRB. After completion of the RRC connection procedure between the userequipment and the base station, the user equipment may be provided bythe base station with the setting information on the primary componentcarrier and the secondary component carrier. The setting information onthe secondary component carrier may include addition/deletion (oractivation/deactivation) of the secondary component carrier. Therefore,in order to activate a secondary component carrier between a basestation and a user equipment or deactivate a previous secondarycomponent carrier, it may be necessary to perform an exchange of RRCsignaling and MAC control element.

The activation or deactivation of the secondary component carrier may bedetermined by a base station based on a quality of service (QoS), a loadcondition of carrier and other factors. And, the base station may beable to instruct a user equipment of secondary component carrier settingusing a control message including such information as an indication type(activation/deactivation) for DL/UL, a secondary component carrier listand the like.

FIG. 7 is a conceptual diagram for dual connectivity (DC) between aMaster Cell Group (MCS) and a Secondary Cell Group (SCG).

The dual connectivity means that the UE can be connected to both aMaster eNode-B (MeNB) and a Secondary eNode-B (SeNB) at the same time.The MCG is a group of serving cells associated with the MeNB, comprisingof a PCell and optionally one or more SCells. And the SCG is a group ofserving cells associated with the SeNB, comprising of the special SCelland optionally one or more SCells. The MeNB is an eNB which terminatesat least S1-MME (S1 for the control plane) and the SeNB is an eNB thatis providing additional radio resources for the UE but is not the MeNB.

The dual connectivity is a kind of carrier aggregation in that the UE isconfigured a plurality serving cells. However, unlike all serving cellssupporting carrier aggregation of FIG. 8 are served by a same eNB, allserving cells supporting dual connectivity of FIG. 10 are served bydifferent eNBs, respectively at same time. The different eNBs areconnected via non-ideal backhaul interface because the UE is connectedwith the different eNBs at same time.

With dual connectivity, some of the data radio bearers (DRBs) can beoffloaded to the SCG to provide high throughput while keeping schedulingradio bearers (SRBs) or other DRBs in the MCG to reduce the handoverpossibility. The MCG is operated by the MeNB via the frequency of f1,and the SCG is operated by the SeNB via the frequency of f2. Thefrequency f1 and f2 may be equal. The backhaul interface (BH) betweenthe MeNB and the SeNB is non-ideal (e.g. X2 interface), which means thatthere is considerable delay in the backhaul and therefore thecentralized scheduling in one node is not possible.

FIG. 8a shows C-plane (Control Plane) connectivity of eNBs involved indual connectivity for a certain UE: The MeNB is C-plane connected to theMME via S1-MME, the MeNB and the SeNB are interconnected via X2-C(X2-Control plane). As FIG. 8a , Inter-eNB control plane signaling fordual connectivity is performed by means of X2 interface signaling.Control plane signaling towards the MME is performed by means of S1interface signaling. There is only one S1-MME connection per UE betweenthe MeNB and the MME. Each eNB should be able to handle UEsindependently, i.e. provide the PCell to some UEs while providingSCell(s) for SCG to others. Each eNB involved in dual connectivity for acertain UE owns its radio resources and is primarily responsible forallocating radio resources of its cells, respective coordination betweenMeNB and SeNB is performed by means of X2 interface signaling.

FIG. 8b shows U-plane connectivity of eNBs involved in dual connectivityfor a certain UE. U-plane connectivity depends on the bearer optionconfigured: i) For MCG bearers, the MeNB is U-plane connected to theS-GW via S1-U, the SeNB is not involved in the transport of user planedata, ii) For split bearers, the MeNB is U-plane connected to the S-GWvia S1-U and in addition, the MeNB and the SeNB are interconnected viaX2-U, and iii) For SCG bearers, the SeNB is directly connected with theS-GW via S1-U. If only MCG and split bearers are configured, there is noS1-U termination in the SeNB. In the dual connectivity, enhancement ofthe small cell is required in order to data offloading from the group ofmacro cells to the group of small cells. Since the small cells can bedeployed apart from the macro cells, multiple schedulers can beseparately located in different nodes and operate independently from theUE point of view. This means that different scheduling node wouldencounter different radio resource environment, and hence, eachscheduling node may have different scheduling results.

FIG. 9 is a conceptual diagram for radio protocol architecture for dualconnectivity.

E-UTRAN of the present example can support dual connectivity operationwhereby a multiple receptions/transmissions (RX/TX) UE in RRC_CONNECTEDis configured to utilize radio resources provided by two distinctschedulers, located in two eNBs (or base stations) connected via anon-ideal backhaul over the X2 interface. The eNBs involved in dualconnectivity for a certain UE may assume two different roles: an eNB mayeither act as the MeNB or as the SeNB. In dual connectivity, a UE can beconnected to one MeNB and one SeNB.

In the dual connectivity operation, the radio protocol architecture thata particular bearer uses depends on how the bearer is setup. Threealternatives exist, MCG bearer (901), split bearer (903) and SCG bearer(905). Those three alternatives are depicted on FIG. 9. The SRBs(Signaling Radio Bearers) are always of the MCG bearer and thereforeonly use the radio resources provided by the MeNB. The MCG bearer (901)is a radio protocol only located in the MeNB to use MeNB resources onlyin the dual connectivity. And the SCG bearer (905) is a radio protocolonly located in the SeNB to use SeNB resources in the dual connectivity.

Specially, the split bearer (903) is a radio protocol located in boththe MeNB and the SeNB to use both MeNB and SeNB resources in the dualconnectivity and the split bearer (903) may be a radio bearer comprisingone Packet Data Convergence Protocol (PDCP) entity, two Radio LinkControl (RLC) entities and two Medium Access Control (MAC) entities forone direction. Specially, the dual connectivity operation can also bedescribed as having at least one bearer configured to use radioresources provided by the SeNB.

The expected benefits of the split bearer (903) are: i) the SeNBmobility hidden to CN, ii) no security impacts with ciphering beingrequired in MeNB only, iii) no data forwarding between SeNBs required atSeNB change, iv) offloads RLC processing of SeNB traffic from MeNB toSeNB, v) little or no impacts to RLC, vi) utilization of radio resourcesacross MeNB and SeNB for the same bearer possible, vii) relaxedrequirements for SeNB mobility (MeNB can be used in the meantime).

The expected drawbacks of the split bearer (903) are: i) need to route,process and buffer all dual connectivity traffic in the MeNB, ii) a PDCPentity to become responsible for routing PDCP PDUs towards eNBs fortransmission and reordering them for reception, iii) flow controlrequired between the MeNB and the SeNB, iv) in the uplink, logicalchannel prioritization impacts for handling RLC retransmissions and RLCStatus PDUs (restricted to the eNB where the corresponding RLC entityresides) and v) no support of local break-out and content caching atSeNB for dual connectivity UEs.

In Dual Connectivity, two MAC entities are configured in the UE: one forthe MCG and one for the SCG. Each MAC entity is configured by RRC with aserving cell supporting PUCCH transmission and contention based RandomAccess. The term SpCell refers to such cell, whereas the term SCellrefers to other serving cells. The term SpCell either refers to thePCell of the MCG or the PSCell of the SCG depending on if the MAC entityis associated to the MCG or the SCG, respectively. A Timing AdvanceGroup containing the SpCell of a MAC entity is referred to as pTAG,whereas the term sTAG refers to other TAGs.

The functions of the different MAC entities in the UE operateindependently if not otherwise indicated. The timers and paramentersused in each MAC entity are configured independently if not otherwiseindicated. The Serving Cells, C-RNTI, radio bearers, logical channels,upper and lower layer entities, LCGs, and HARQ entities considered byeach MAC entity refer to those mapped to that MAC entity if nototherwise indicated

On the other hand, in the dual connectivity, one PDCP entity isconfigured in the UE. For one UE, there are two different eNBs that areconnected via non-ideal backhaul X2. In case the split bearer (903) istransmitted to different eNBs (MeNB and SeNB), the SeNB forwards thePDCP PDU to the MeNB. Due to the delay over non-ideal backhaul, the PDCPPDUs are likely to be received out-of-sequence.

FIG. 10 is a diagram for a general overview of the LTE protocolarchitecture for the downlink.

A general overview of the LTE protocol architecture for the downlink isillustrated in FIG. 10. Furthermore, the LTE protocol structure relatedto uplink transmissions is similar to the downlink structure in FIG. 10,although there are differences with respect to transport formatselection and multi-antenna transmission.

Data to be transmitted in the downlink enters in the form of IP packetson one of the SAE bearers (1001). Prior to transmission over the radiointerface, incoming IP packets are passed through multiple protocolentities, summarized below and described in more detail in the followingsections:

Packet Data Convergence Protocol (PDCP, 1003) performs IP headercompression to reduce the number of bits necessary to transmit over theradio interface. The header-compression mechanism is based on ROHC, astandardized header-compression algorithm used in WCDMA as well asseveral other mobile-communication standards. PDCP (1003) is alsoresponsible for ciphering and integrity protection of the transmitteddata. At the receiver side, the PDCP protocol performs the correspondingdeciphering and decompression operations. There is one PDCP entity perradio bearer configured for a mobile terminal.

Radio Link Control (RLC, 1005) is responsible forsegmentation/concatenation, retransmission handling, and in-sequencedelivery to higher layers. Unlike WCDMA, the RLC protocol is located inthe eNodeB since there is only a single type of node in the LTEradio-access-network architecture. The RLC (1005) offers services to thePDCP (1003) in the form of radio bearers. There is one RLC entity perradio bearer configured for a terminal.

There is one RLC entity per logical channel configured for a terminal,where each RLC entity is responsible for: i) segmentation,concatenation, and reassembly of RLC SDUs; ii) RLC retransmission; andiii) in-sequence delivery and duplicate detection for the correspondinglogical channel.

Other noteworthy features of the RLC are: (1) the handling of varyingPDU sizes; and (2) the possibility for close interaction between thehybrid-ARQ and RLC protocols. Finally, the fact that there is one RLCentity per logical channel and one hybrid-ARQ entity per componentcarrier implies that one RLC entity may interact with multiplehybrid-ARQ entities in the case of carrier aggregation.

The purpose of the segmentation and concatenation mechanism is togenerate RLC PDUs of appropriate size from the incoming RLC SDUs. Onepossibility would be to define a fixed PDU size, a size that wouldresult in a compromise. If the size were too large, it would not bepossible to support the lowest data rates. Also, excessive padding wouldbe required in some scenarios. A single small PDU size, however, wouldresult in a high overhead from the header included with each PDU. Toavoid these drawbacks, which is especially important given the verylarge dynamic range of data rates supported by LTE, the RLC PDU sizevaries dynamically.

In process of segmentation and concatenation of RLC SDUs into RLC PDUs,a header includes, among other fields, a sequence number, which is usedby the reordering and retransmission mechanisms. The reassembly functionat the receiver side performs the reverse operation to reassemble theSDUs from the received PDUs.

Medium Access Control (MAC, 1007) handles hybrid-ARQ retransmissions anduplink and downlink scheduling. The scheduling functionality is locatedin the eNodeB, which has one MAC entity per cell, for both uplink anddownlink. The hybrid-ARQ protocol part is present in both thetransmitting and receiving end of the MAC protocol. The MAC (1007)offers services to the RLC (1005) in the form of logical channels(1009).

Physical Layer (PHY, 1011), handles coding/decoding,modulation/demodulation, multi-antenna mapping, and other typicalphysical layer functions. The physical layer (1011) offers services tothe MAC layer (1007) in the form of transport channels (1013).

FIG. 11 is a conceptual diagram for a PDCP entity architecture.

FIG. 11 represents one possible structure for the PDCP sublayer, but itshould not restrict implementation. Each RB (i.e. DRB and SRB, exceptfor SRB0) is associated with one PDCP entity. Each PDCP entity isassociated with one or two (one for each direction) RLC entitiesdepending on the RB characteristic (i.e. uni-directional orbi-directional) and RLC mode. The PDCP entities are located in the PDCPsublayer. The PDCP sublayer is configured by upper layers.

FIG. 12 is a conceptual diagram for functional view of a PDCP entity.

The PDCP entities are located in the PDCP sublayer. Several PDCPentities may be defined for a UE. Each PDCP entity carrying user planedata may be configured to use header compression. Each PDCP entity iscarrying the data of one radio bearer. In this version of thespecification, only the robust header compression protocol (ROHC), issupported. Every PDCP entity uses at most one ROHC compressor instanceand at most one ROHC decompressor instance. A PDCP entity is associatedeither to the control plane or the user plane depending on which radiobearer it is carrying data for.

FIG. 12 represents the functional view of the PDCP entity for the PDCPsublayer, it should not restrict implementation. For RNs, integrityprotection and verification are also performed for the u-plane.

At reception of a PDCP SDU from upper layers, the UE may start a discardtimer associated with the PDCP SDU. For a PDCP SDU received from upperlayers, the UE may associate a PDCP SN (Sequence Number) correspondingto Next_PDCP_TX_SN to the PDCP SDU (S1201), perform header compressionof the PDCP SDU (S1203), perform integrity protection (S1205) andciphering using COUNT based on TX_HFN and the PDCP SN associated withthis PDCP SDU (S1207), increment the Next_PDCP_TX_SN by one, and submitthe resulting PDCP Data PDU to lower layer (S1209).

If the Next_PDCP_TX_SN is greater than Maximum_PDCP_SN, theNext_PDCP_TX_SN is set to ‘0’ and TX_HFN is incremented by one.

When the discard timer expires for a PDCP SDU, or the successfuldelivery of a PDCP SDU is confirmed by PDCP status report, the UE maydiscard the PDCP SDU along with the corresponding PDCP PDU. If thecorresponding PDCP PDU has already been submitted to lower layers thediscard is indicated to lower layers. The transmitting side of each PDCPentity for DRBs may maintain the discard timer. The duration of thetimer is configured by upper layers. In the transmitter, a new timer isstarted upon reception of an SDU from upper layer.

In dual connectivity, for one UE, there are two different eNBs that areconnected via non-ideal backhaul X2. In case the split bearer istransmitted to different eNBs (MeNB and SeNB), the SeNB forwards thePDCP PDU to the MeNB. Due to the delay over non-ideal backhaul, the PDCPPDUs are likely to be received out-of-sequence. Therefore, re-orderingfunction is needed in the PDCP receiver side to rearrange the processingorder of PDCP PDUs in sequential order.

If the PDCP reordering function is operated based on the reorderingtimer as in the RLC reordering function, the reordering timer startsupon detection of out-of-sequence PDCP PDU and limit the maximum waitingtime for the missing PDCP PDU. Given that PDCP SDU is discarded uponPDCP SDU discard timer expiry at the transmitter side, there is a casethat PDCP reordering timer starts upon detection of out-of-sequence PDCPPDU which is already discarded in the PDCP transmitter side.Consequently, the PDCP receiver in the UE side would hold re-assembly ofPDCP SDUs although the missing PDCP PDU can never be received. This hasnegative impact on throughput performance, which is the key goal of dualconnectivity.

FIG. 13 is a conceptual diagram for processing a PDCP SDU in a dualconnectivity system according to embodiments of the present invention.

In this invention, in order to avoid delaying re-assembly of PDCP SDUs(Packet Data Convergence Protocol Service Data Unit) in the PDCPreceiver due to the discarded PDCP SDU in the PDCP transmitter, the PDCPtransmitter avoids PDCP Sequence Number gap in the transmitted PDCPPDUs. For this purpose, the PDCP transmitter keeps transmitting PDCP SDUeven if a discard timer expires for the PDCP SDU if the PDCP SDU isalready associated with a PDCP Sequence Number.

At reception of a PDCP SDU from upper layers (S1301), the PDCPtransmitter starts a discard timer associated with the PDCP SDU (S1303).

The PDCP transmitter associates a PDCP sequence number to the PDCP SDU(S1305), generates a PDCP PDU using the PDCP SDU and the PDCP SequenceNumber associated with the PDCP SDU (S1307), and submits the generatedPDCP PDU to a lower layer (S1309).

The PDCP transmitter can check whether a certain condition related tothe PDCP SDU process is satisfied or not when the discard timer startedin the step of S1303 expires. Because who does not know when the discardtimer expires, the certain condition related to the PDCP SDU process canbe differently determined.

[Case 1]

If the discard timer expires during the step of S1305, the PDCPtransmitter checks whether the PDCP SDU is associated with a PDCPSequence Number or not (S1304). In this case, the certain conditionrelated to the PDCP SDU process is satisfied if the PDCP sequence numberis not associated with the PDCP SDU when the timer expires.

Thus, if the PDCP SDU is already associated with the PDCP SequenceNumber when the discard timer expires (S1305), the PDCP transmitterperforms transmission of the PDCP SDU, (i.e. header compression andciphering). And the PDCP transmitter generates a PDCP PDU (Protocol DataUnit) using the PDCP SDU and the PDCP SN associated with the PDCP SDU(S1307), and submits to RLC layer (S1309). The PDCP transmitteroptionally discards the corresponding PDCP SDU.

Otherwise, if the PDCP sequence number is not associated with the PDCPSDU when the timer expires, the PDCP transmitter discards thecorresponding PDCP SDU (S1311).

[Case 2]

If the discard timer expires during the step of S1307, the PDCPtransmitter checks whether a PDCP PDU including the PDCP SDU and PDCP SNassociated with the PDCP SDU is generated or not (S1306). In this case,the certain condition related to the PDCP SDU process is satisfied ifthe PDCP PDU including the PDCP SDU and PDCP SN associated with the PDCPSDU is not generated when the timer expires.

If the PDCP PDU including the PDCP SDU and the associated PDCP SequenceNumber is already generated (S1307), the PDCP transmitter submits thegenerated PDCP PDU to the RLC layer (S1309). The PDCP transmitteroptionally discards the corresponding PDCP SDU.

Otherwise, if the PDCP PDU including the PDCP SDU and the associatedPDCP Sequence Number has not been generated, the PDCP transmitterdiscards the corresponding PDCP SDU (S1311). The PDCP transmitterperforms renumbering of the PDCP SDUs that follow the discarded PDCP SDUso that there is no PDCP Sequence Number gap between transmitted PDCPSDUs (S1313). For example, if PDCP SDU with PDCP SN=3 is discarded, allthe following PDCP SDUs, i.e. PDCP SN=4, 5, 6, . . . are renumbered toPDCP SN=3, 4, 5, . . .

[Case 3]

If the discard timer expires during the step of S1309, the PDCPtransmitter checks whether a PDCP PDU including the PDCP SDU and PDCP SNassociated with the PDCP SDU is submitted or not (S1308). In this case,the certain condition related to the PDCP SDU process is satisfied ifthe PDCP PDU including the PDCP SDU and PDCP SN associated with the PDCPSDU is not submitted to a lower layer when the timer expires.

If the PDCP PDU including the PDCP SDU and the associated PDCP SequenceNumber is already submitted to the RLC layer, the PDCP transmitter doesnothing (S1309). The PDCP transmitter optionally discards thecorresponding PDCP SDU.

If the PDCP PDU including the PDCP SDU and the associated PDCP SequenceNumber has not been submitted to the RLC layer, the PDCP transmitterdiscards the corresponding PDCP PDU and PDCP SDU (S1311). The PDCPtransmitter performs renumbering of the PDCP SDUs that follow thediscarded PDCP SDU so that there is no PDCP Sequence Number gap betweentransmitted PDCP SDUs (S1313). For example, if PDCP SDU with PDCP SN=3is discarded, all the following PDCP SDUs, i.e. PDCP SN=4, 5, 6, . . .are renumbered to PDCP SN=3, 4, 5, . . .

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the spirit or scope of the invention. Thus, it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

The embodiments of the present invention described hereinbelow arecombinations of elements and features of the present invention. Theelements or features may be considered selective unless otherwisementioned. Each element or feature may be practiced without beingcombined with other elements or features. Further, an embodiment of thepresent invention may be constructed by combining parts of the elementsand/or features. Operation orders described in embodiments of thepresent invention may be rearranged. Some constructions of any oneembodiment may be included in another embodiment and may be replacedwith corresponding constructions of another embodiment. It is obvious tothose skilled in the art that claims that are not explicitly cited ineach other in the appended claims may be presented in combination as anembodiment of the present invention or included as a new claim bysubsequent amendment after the application is filed.

In the embodiments of the present invention, a specific operationdescribed as performed by the BS may be performed by an upper node ofthe BS. Namely, it is apparent that, in a network comprised of aplurality of network nodes including a BS, various operations performedfor communication with an MS may be performed by the BS, or networknodes other than the BS. The term ‘eNB’ may be replaced with the term‘fixed station’, ‘Node B’, ‘Base Station (BS)’, ‘access point’, etc.

The above-described embodiments may be implemented by various means, forexample, by hardware, firmware, software, or a combination thereof.

In a hardware configuration, the method according to the embodiments ofthe present invention may be implemented by one or more ApplicationSpecific Integrated Circuits (ASICs), Digital Signal Processors (DSPs),Digital Signal Processing Devices (DSPDs), Programmable Logic Devices(PLDs), Field Programmable Gate Arrays (FPGAs), processors, controllers,microcontrollers, or microprocessors.

In a firmware or software configuration, the method according to theembodiments of the present invention may be implemented in the form ofmodules, procedures, functions, etc. performing the above-describedfunctions or operations. Software code may be stored in a memory unitand executed by a processor. The memory unit may be located at theinterior or exterior of the processor and may transmit and receive datato and from the processor via various known means.

Those skilled in the art will appreciate that the present invention maybe carried out in other specific ways than those set forth hereinwithout departing from the spirit and essential characteristics of thepresent invention. The above embodiments are therefore to be construedin all aspects as illustrative and not restrictive. The scope of theinvention should be determined by the appended claims and their legalequivalents, not by the above description, and all changes coming withinthe meaning and equivalency range of the appended claims are intended tobe embraced therein.

INDUSTRIAL APPLICABILITY

While the above-described method has been described centering on anexample applied to the 3GPP LTE system, the present invention isapplicable to a variety of wireless communication systems in addition tothe 3GPP LTE system.

1. A method for an User Equipment (UE) operating in a wirelesscommunication system, the method comprising: receiving a PDCP SDU(Packet Data Convergence Protocol Service Data Unit) from a upper layer;starting a timer associated with the PDCP SDU; checking whether acertain condition related to the PDCP SDU process is satisfied or notwhen the timer expires; and discarding the PDCP SDU if the certaincondition related to the PDCP SDU process is satisfied.
 2. The methodaccording to claim 1, wherein the certain condition related to the PDCPSDU process is satisfied if the PDCP sequence number is not associatedwith the PDCP SDU when the timer expires.
 3. The method according toclaim 2, the method further comprising: generating a PDCP PDU (ProtocolData Unit) using the PDCP SDU and the PDCP Sequence Number associatedwith the PDCP SDU, if the PDCP Sequence Number is associated with thePDCP SDU when the timer expires.
 4. The method according to claim 3, themethod further comprising: submitting the generated PDCP PDU to a lowerlayer.
 5. The method according to claim 1, the certain condition relatedto the PDCP SDU process is satisfied if a PDCP PDU (Protocol Data Unit)including the PDCP SDU and PDCP SN associated with the PDCP SDU is notgenerated when the timer expires.
 6. The method according to claim 5,the method further comprising: performing a re-numbering of consecutivePDCP SDUs that follow the discarded PDCP SDU so that there is no PDCPSequence Number gap between the consecutive PDCP SDUs.
 7. The methodaccording to claim 1, the certain condition related to the PDCP SDUprocess is satisfied if the PDCP PDU (Protocol Data Unit) including thePDCP SDU and PDCP SN associated with the PDCP SDU is not submitted tothe lower layer when the timer expires.
 8. The method according to claim7, the method further comprising: performing a re-numbering ofconsecutive PDCP SDUs that follow the discarded PDCP SDU so that thereis no PDCP Sequence Number gap between the consecutive PDCP SDUs.
 9. AUE (User Equipment) for operating in a wireless communication system,the UE comprising: an RF (Radio Frequency) module; and a processorconfigured to control the RF module, wherein the processor is configuredto receive a PDCP SDU (Packet Data Convergence Protocol Service DataUnit) from a upper layer, to start a timer associated with the PDCP SDU,to check whether a certain condition related to the PDCP SDU process issatisfied or not when the timer expires, and to discard the PDCP SDU ifthe certain condition related to the PDCP SDU process is satisfied. 10.The UE according to claim 9, the certain condition related to the PDCPSDU process is satisfied if the PDCP Sequence Number is not associatedwith the PDCP SDU when the timer expires.
 11. The UE according to claim10, wherein the processor is further configured to generate a PDCP PDU(Protocol Data Unit) using the PDCP SDU and the PDCP Sequence Numberassociated with the PDCP SDU, if the PDCP Sequence Number is associatedwith the PDCP SDU when the timer expires.
 12. The UE according to claim11, wherein the processor is further configured to submit the generatedPDCP PDU to a lower layer.
 13. The UE according to claim 9, the certaincondition related to the PDCP SDU process is satisfied if a PDCP PDU(Protocol Data Unit) including the PDCP SDU and PDCP SN associated withthe PDCP SDU is not generated when the timer expires.
 14. The UEaccording to claim 13, wherein the processor is further configured toperform a re-numbering of consecutive PDCP SDUs that follow thediscarded PDCP SDU so that there is no PDCP Sequence Number gap betweenthe consecutive PDCP SDUs.
 15. The UE according to claim 9, the certaincondition related to the PDCP SDU process is satisfied if the PDCP PDU(Protocol Data Unit) including the PDCP SDU and PDCP SN associated withthe PDCP SDU is not submitted to the lower layer when the timer expires.16. The UE according to claim 15, wherein the processor is furtherconfigured to perform a re-numbering of consecutive PDCP SDUs thatfollow the discarded PDCP SDU so that there is no PDCP Sequence Numbergap between the consecutive PDCP SDUs.